There are many applications wherein light influencing displays are utilized to advantage. For example, light influencing displays find use in digital watches, digital clocks, calculators, pocket sized television receivers, and various forms of portable games, to name just a few.
Light influencing displays can be formed in many configurations. By the term "light influencing material" is meant any material which emits light or can be used to selectively vary the intensity, phase, or polarization of light either being reflected from or transmitted through the material. Liquid crystal material is only one such material having these characteristics. Generally, each pixel includes a pair of electrodes which can be individually addressable. As is well known, when a voltage is applied across the electrodes, the optical properties of the light influencing material can be changed to provide a light or dark display depending upon the type of material used and the desired mode of operation of the display.
An increasingly important type of light influencing display matrix includes a large number of pixel elements arranged in rows and columns. Because of the large number of pixels in the matrix arrays, the electrode line connections to each pixel are on common lines with other pixels. In this so-called multiplexing scheme, each pixel is located at a unique intersection of two address lines. The pixels are individually addressed by applying a voltage potential across its two intersecting lines. The utilization of this multiplexing scheme relies upon the innate voltage threshold characteristic of the display material, which provides an optical change only for applied potentials greater than the threshold voltage. Thus, pixels can experience an increased voltage potential, because they are coupled to one of the address lines with an applied potential, but they will not be activated because the potential increase caused by the potential on one line is below the threshold voltage of the pixel.
Matrix light influencing displays, such as liquid crystal displays which rely only upon the innate liquid crystal display threshold voltage to differentiate the applied voltage potentials are limited in the number of pixels, contrast and speed because of the finite sharpness of the threshold voltage. To achieve high resolution liquid crystal display matrices with large numbers of pixels with acceptable contrast and speed, it is necessary to include an additional isolation device at each pixel to provide adequate isolation from potentials applied to other pixels on the common address lines. These so-called active matrix liquid crystal displays utilize a number of types of two or three terminal isolation devices to provide the required isolation from the applied voltage potentials. By the term "isolation" is meant any device which enhances the ability for one pixel to be addressed (switched) without switching other pixels on a common address line. Such "isolation" can include threshold devices such as diodes in various configurations, M-I-M structures, etc., which provide a more precise voltage threshold than that provided by the light influencing material itself. A more precise voltage threshold means a smaller variance in the voltage (smaller .DELTA.v) required to switch the pixel from off to on. Other examples of isolation devices can include switching devices, such as thin film transistors, which can have a very small .DELTA.v.
Some two terminal isolation devices, such as diodes and some configurations of three terminal devices can be thought of as single polarity devices, which can be turned on in only one direction or polarity. Three terminal devices, such as thin film transistors and other two terminal devices, such as diode rings, threshold switches, metal-insulator-metal (M-I-M) devices and N.sup.+ -I-N.sup.+ devices, can be thought of as dual polarity devices which can be turned on in two directions or polarities.
One problem in using active matrix light influencing displays is yield. Virtually one-hundred percent of all of the isolation devices must be operational to obtain a useable display. Such yields can be difficult to achieve for large area displays, because the making of active matrix displays requires numerous process steps, many of which can require extremely accurate photolithography.
Diodes suitable for isolation devices in active matrix display applications are disclosed in U.S. application Ser. No. 573,004, entitled "Liquid Crystal Displays Operated By Amorphous Silicon Alloy Diodes", in the names of Zvi Yaniv, Vincent D. Cannella, Gregory L. Hansell and Louis D. Schwartz, filed Jan. 23, 1984, which is incorporated herein by reference. The diodes can be formed without the need of precise photolithography and in fewer process steps than that required to form some of the prior isolation devices.
The individual pixel structures in prior light influencing displays result in lower frequency operation, more complex electronic circuitry, less flexibility, reduced yield and less isolation than desired. Whether utilizing diodes or transistors as isolation devices, the prior circuits have the pixel isolation devices, one address line and one electrode on one substrate or plane and the other electrode and address line on the other plane. In effect, the display material and the two electrodes form a capacitor which limits the operational frequency. The electronic circuitry is more complex than desired and limits the flexibility of the displays, since both planes have circuitry thereon. Where there is no redundancy in the pixel isolation devices, any one inoperative device will cause an inoperative pixel element reducing the display yield. Further, the prior display pixels in attempting to reduce the circuitry on both planes, generally are limited to a pair of electrodes with the isolation device or devices only on one plane and coupled to only one electrode address line on that plane.